The resin flux cored solder is a material made of a solder alloy having a hollow filled with a solid or high viscosity liquid flux. The flux coated solder is a material of a solder alloy whose outer surface is coated with a flux. The flux in resin flux cored solder or flux coated solder removes chemically a metal oxide present on the solder and the metal surface to be soldered at the time of soldering to enable a metal element to move in the boundary between the both. By using the flux, an intermetallic compound is formed between the solder and the metal surface to be soldered, so that strong joint may be obtained.
When parts are joined together using a resin flux cored solder or a flux coated solder, a scattering of flux, and a scattering of solder entrained therewith may occur. Since the scattering of flux and solder may lead to short circuit between electrodes of the joined parts, and contamination of the substrate, it is required to prevent the scattering of flux and solder.
For a resin flux cored solder or a flux coated solder, which comprises a flux, the flux is required to be in a solid or high viscosity liquid state at room temperature (25° C.) from the viewpoint of a processability. When a flux is in a low viscosity liquid state, it becomes difficult to process (process to an arbitrary size and shape) a resin flux cored solder or a flux coated solder.
As a conventional flux for a resin flux cored solder, for example, a flux comprising an inhibitor of concentrated gas release, by generating a large number of fine microbubbles in a flux molten by heating at the time of soldering so as to permit a plurality of minute gas escapes has been proposed (PTL 1). In the flux described in PTL 1, an acid-modified rosin, or an organic acid, which is a kind of activator, is used as an inhibitor of concentrated gas release, by which a minute gas component is discharged quickly so that generation of a large volume gas component leading to the scattering of flux may be prevented. However, in the flux described in PTL 1, a rosin resin and an activator, which are main components of the flux, are limited to specific components, and therefore there appear restrictions on optimization of the components of flux, such as a rosin resin and an activator, according to the type of a solder alloy or joining conditions.
In addition, for preventing the scattering of flux, for example, a solder composition comprising a flux comprising a rosin resin, an activator, a solvent, and an antifoaming agent having a solubility parameter (SP value) of 9.5 or less, and a weight average molecular weight of 100000 or more, and a solder powder has been proposed (PTL 2). However, PTL 2 is designed to prevent the scattering of flux in a solder composition (solder paste) composed of a mixture of a flux and a solder powder by using an antifoaming agent having a specific weight average molecular weight. But, PTL 2 is not designed to prevent the scattering of flux in a resin flux cored solder or a flux coated solder, in which a flux and a solder are not mixed and exist separately. Further, since the flux in PTL 2 comprises a solvent, and is in a low viscosity liquid state, it is difficult to process a resin flux cored solder or a flux coated solder comprising such a flux.
Further, it has been proposed that a leveling agent, an antifoaming agent, and a solvent are also blended into a flux composition for soldering comprising a resin obtained by a ring-opening half esterification reaction of a cyclic acid anhydride having a 6-membered carbon ring structure with a polyhydric alcohol, and a specific silicon compound (PTL 3). However, PTL 3 is a technique relevant to a solder paste in which a flux and a solder powder are mixed. But, PTL 3 is not relevant to a resin flux cored solder or a flux coated solder, in which a flux and a solder are not mixed and exist separately. Further, PTL 3 does not disclose a technique of prevention of scattering of flux by using a leveling agent and an antifoaming agent. Meanwhile, a flux in PTL 3 comprises a solvent, and is in a state of low viscosity liquid, and therefore it is difficult to process a resin flux cored solder, or a flux coated solder comprising such a flux.
As described above, a resin flux cored solder or a flux coated solder, for which a scattering of flux and solder at the time of using the solder is suppressed, and which is superior in a processability, and a flux to be contained therein have been demanded.